A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is a key component (memory element) in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic read head. A TMR sensor typically has a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic insulator layer. The sensor stack in a so-called bottom spin valve configuration is generally comprised of a seed (buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer, tunnel barrier layer, free layer, and capping layer that are sequentially formed on a substrate. The free layer serves as a sensing layer that responds to external fields (media field) while the pinned layer is relatively fixed and functions as a reference layer. The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield. When a sense current is passed from the top shield to the bottom shield (or top conductor to bottom conductor in a MRAM device) in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
A giant magnetoresistive (GMR) head is another type of memory device. In this design, the insulator layer between the pinned layer and free layer in the TMR stack is replaced by a non-magnetic conductive layer such as copper.
In the TMR stack, the pinned layer may have a synthetic anti-ferromagnetic (SyAF) configuration in which an outer pinned layer is magnetically coupled through a coupling layer to an inner pinned layer that contacts the tunnel barrier. The outer pinned layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer which is magnetized in the same direction. The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
A TMR sensor is currently the most promising candidate for replacing a GMR sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantage of a TMR sensor is that a substantially higher MR ratio can be realized than for a GMR sensor. In addition to a high MR ratio, a high performance TMR sensor requires a low areal resistance RA (area×resistance) value, a free layer with low magnetostriction (λ) and low coercivity (Hc), a strong pinned layer, and low interlayer coupling (Hin) through the barrier layer. The MR ratio (also referred to as TMR ratio) is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um2.
A MgO based MTJ is a very promising candidate for high frequency recording applications because its tunneling magnetoresistive (TMR) ratio is significantly higher than for AlOx or TiOx based MTJs. S. Yuasa et al. in “Giant room-temperature magnetoresistance in single crystal Fe/MgO/Fe magnetic tunnel junctions”, Nature Materials 3, 868-871 (2004) and S. Parkin et al. in “Giant tunneling magnetoresistance at room temperature with MgO (100) tunnel barriers”, Nature Materials 3, 862-867 (2004) demonstrated that a MR ratio of ˜200% can be achieved at room temperature in epitaxial Fe(001)/MgO(001)/Fe(001) and in polycrystalline FeCo(001)/MgO(001)/(Fe70CO30)80B20 MTJs. Yuasa et al. reported an MR ratio as high as 410% at RT in “Giant tunneling magnetoresistance up to 410% at room temperature in fully epitaxial Co/MgO/Co magnetic tunnel junctions with bcc Co(001) electrodes”, Applied Physics Letters, 89, 042505 (2006). Meanwhile, D. Djayaprawira et. al showed that MTJs of CoFeB/MgO(001)/CoFeB structure made by conventional sputtering can also have a very high MR ratio of 230% with advantages of better flexibility and uniformity in “230% room temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions”, Physics Letters 86, 092502 (2005). However, the aforementioned references have a higher than desired RA value.
For a low RA application, the MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4 ohm/μm2 according to K. Tsunekawa et al. in “Giant tunneling magnetoresistance effect in low resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read head applications”, Applied Physics Letters 87, 072503 (2005). In this case, a DC-sputtered Mg layer was inserted between the CoFeB pinned layer and an RF-sputtered MgO layer, an idea initially proposed by T. Linn et al. in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO(reactive sputtering)/NiFe structure. Also, a Ta getter presputtering prior to RF sputtering a MgO layer can achieve 55% TMR with 0.4 ohm/μm2 as reported by Y. Nagamine et al. in “Ultralow resistance-area product of 0.4 ohm/μm2 and high magnetoresistance above 50% in CoFeB/MgO/CoFeB magnetic junctions”, Applied Physics Letters, 89, 162507 (2006).
An alternative method of forming low RA in a MTJ with a MgO tunnel barrier is to DC sputter deposit a first Mg layer, perform a natural oxidation (NOX) process, and then DC sputter deposit a second Mg layer on the resulting MgO layer as described in related U.S. Patent Application Publication 2009/0229111. Benefits include better process control and improved MRR (read) uniformity.
In order to achieve a smaller Hc but still maintain a high TMR ratio, the industry tends to use CoFeB as the free layer in a TMR sensor based on a MgO tunnel barrier. Insertion of a thin CoFe layer between a MgO barrier and CoFeB free layer may be used for realizing a high MR ratio at a low annealing temperature below 300° C. However, there are two major concerns associated with a CoFeB free layer. One is a high positive magnetostriction (λ) and a second issue is a CoFeB free layer tends to cause excessive noise and lower the signal to noise ratio (SNR) which is undesirable. Furthermore, advanced technology requires an improved free layer in a TMR sensor with a TMR ratio above 60% and an RA of about 1.5 ohm/μm2 or less in addition to a low value for magnetostriction, Hc, and noise level.
U.S. Patent Application Publication No. 2005/0052793 teaches a free layer with a trilayer configuration wherein each of the first, second, and third layers is selected from a group including Ni, Co, Fe, B, CoFe, CoFeB, NiFe, and alloys thereof.
U.S. Patent Application Publication No. 2003/0123198 discloses a free layer that may be made of CoFe, NiFe, CoFeB, or a lamination layer film thereof to realize a larger MR ratio and soft magnetic characteristics.